Directed self-assembly (“DSA”) processes use block copolymers (BCPs) to form lithographic structures, which are formed by the rearrangement of the BCP from a random, unordered state to a structured, ordered state. The morphology of the ordered state is variable and depends on a number of factors, including the relative molecular weight ratios of the block polymers, as well as the surrounding chemical and physical environment. Common morphologies include line-space and cylindrical, although other structures may also be used. For example, other ordered morphologies include spherical, lamellar, bicontinuous gyroid, or miktoarm star microdomains.
Two common methods used to guide self-assembly in BCP thin films are graphical epitaxy (also called “grapho-epitaxy”) and chemical epitaxy (also called “chemo-epitaxy”). In the grapho-epitaxy method, self-organization of block copolymers is guided by pre-patterned substrates. Self-aligned lamellar BCPs can form parallel line-space patterns of different domains in topographical trenches and enhance pattern resolution by subdividing the space of topographical patterns. However, defects and line-edge roughness are easily induced in this grapho-epitaxy directed self-assembly scheme. For example, if the sidewalls are neutral, the lamellae tend to orient perpendicular to the sidewalls and will not subdivide the pitch along the desired direction.
In the chemo-epitaxy method, the self-assembly of BCP domains is guided by chemical patterns having pitch dimensions commensurate with the domain size or pitch period (L0) of the self-assembled BCP morphology. The affinity between the chemical patterns and at least one of the types of BCP domains results in the precise placement of the different BCP domains on respective corresponding regions of the chemical patterns, i.e., a pinning region. The affinity for the one type of domain (for example the A domains of an A-B diblock copolymer assembly) dominates the interaction of the other domain(s) (for example the B domains) with the non-patterned regions of the surface, which can be selective or non-selective (i.e., neutral) towards the other type(s) of domains. As a result, the pattern formation in the resulting BCP assembly can directly mirror the underlying chemical pattern (i.e., can be a one-for-one reproduction of the features of the chemical pre-pattern). Moreover, depending on the domain size or pitch period (L0) of the self-assembled BCP morphology and the critical dimension (CD) of the pinning regions and the non-patterned regions, frequency multiplication can be achieved. However, dimension control and line-edge roughness can be negatively affected in chemo-epitaxy DSA methods by topographical variations in the chemical pre-pattern.
Two commonly used methods for forming chemical pre-patterns involve the removal of a photoresist feature during the formation of the chemical guide. If the removal of the photoresist feature is improperly performed, it may introduce topographical variations in the chemical prepattern, which in turn can negatively impact the DSA process. Furthermore, the removal of the photoresist can also change the surface properties of the chemical guide which may also adversely impact the DSA process.
The first commonly used method is known to those skilled in the art as the “Wisconsin Flow” or “LiNe flow”. Referring to FIGS. 1A-1G, a layered substrate 100a of the prior art is provided having a substrate 101 sequentially layered with a cross-linked polystyrene layer 102 and a patterned layer of photoresist 103 overlying the cross-linked polystyrene layer 102. The imaged layer of photoresist has photoresist lines 104 and openings 106. An oxygen plasma etch step breaks through the cross-linked polystyrene layer 102 and trims the remaining features to provide fine line features 108 and enlarged openings 110, in layered substrate 100b, as shown in FIG. 1B. The fine line features 108 comprise a photoresist portion 108a atop a cross-linked polystyrene portion 108b. Referring to layered substrates 100c-100e in FIGS. 1C-1E, respectively, following a selective removal of the photoresist portion 108a, a layer of hydroxyl terminated poly(styrene-random-methyl methacrylate) (referred to below as the “brush”) 112 is coated over the cross-linked polystyrene portion 108b in planarizing fashion (FIG. 1D), and subsequent baking allows the brush to chemically graft to portions of the substrate 101 that were exposed in the enlarged openings 110. The excess brush material that has planarized the cross-linked polystyrene portions 108b is rinsed off using an appropriate solvent leaving only the portion of the brush that was previously grafted to the substrate, referred to hereafter as remaining neutral layer portions 112. The resulting structure is a nearly planar chemo-epitaxy prepattern. A layer of a block copolymer 116 is applied next (FIG. 1F) and annealed to induce self-assembly to form a lamellar film 118 comprising a first domain 118a and a second 118b (FIG. 1G). One challenging step in the Wisconsin Flow is the removal of the photoresist layer 108a without damaging the underlying polystyrene layer 108b. One method for selectively removing the photoresist portions utilizes a warm, ultrasonic solution of N-methypyrrolidinone (NMP) to strip the resist, but this solution has a number of issues for mainstream manufacturing.
The second commonly used method is known to those skilled in the art as the “IBM Liftoff Flow.” Referring to FIGS. 2A-2E, a layered substrate 200a of the prior art is provided having a substrate 201 coated with an under-layer 202 and a patterned layer of photoresist 203 having imaged regions 204 (from imaging 206) and un-imaged regions 205. Where the layer of photoresist 203 is a positive tone photoresist comprising a photoacid generator, imaged regions 204 are rendered soluble to positive tone developing chemistry, such as aqueous tetramethylammonium hydroxide (TMAH), upon performing a post-exposure bake. As shown in FIG. 2B, after a post-exposure bake, followed by a positive tone development process, the openings 207 are shown on substrate 200b, along with the un-imaged regions 205. A flood exposure step followed by a bake step provides resist lines 208 that consist of deprotected photoresist polymer, shown in FIG. 2C. The resist lines 208 of FIG. 2C are subsequently coated with a cross-linked neutral layer 212, as shown in FIG. 2D. The underlying resist lines 208a of FIG. 2D are then lifted off by exposure to a developer solution, which penetrates the thin cross-linked neutral layer 212, and then dissolves the underlying resist lines 208a. As the resist lines 208a dissolve, the portion of the cross-linked neutral layer 212 attached to the resist lines 208a is essentially lifted off the layered substrate 200d because it has lost its underlying support, i.e., the resist lines 208a, to provide openings 217 between remaining neutral layer portions 216 in the IBM Flow chemo-epitaxy pre-pattern shown in FIG. 2E. Similar to FIGS. 1F-1G, a layer of a block copolymer may be applied and annealed to induce self-assembly to form a lamellar film (not shown). One challenging step in the IBM Flow is the lift-off step. The lift off processing fluid has traditionally been a TMAH developer, in which the cross-linked neutral layer is not soluble, which raises a defectivity concern.
Therefore, due to the aforementioned limitations, new methods for removing photoresist features in chemo-epitaxy prepatterns are highly desirable.